1. Field of the Invention
The present invention relates to exposure control techniques in image capture apparatuses.
2. Description of the Related Art
In image capture apparatuses that use image sensors, such as digital cameras, exposure control is carried out on the image sensor for a variety of purposes and applications. An aperture mechanism that adjusts the exposure amount of the image sensor, an electronic shutter mechanism that adjusts the charge accumulation period of the image sensor, a gain mechanism that adjusts the level of the signal read out from the image sensor, and so on exist as mechanisms used in exposure control.
In the case where the brightness of an object has changed while a continuous image generated by the image capture apparatus is being displayed in an image display device such as an LCD or the like (this is called “live view” hereinafter), the stated mechanisms are used to carry out exposure control for the image sensor so as to maintain an appropriate level of exposure in the image. There are also cases where exposure control is carried out intentionally as part of the composition, such as when the depth of field is changed using the aperture value, the movement of an object is captured using the shutter speed as an element of artistic expression, and so on. Furthermore, there are cases where aperture control is carried out in order to obtain a depth of field necessary for AF, which focuses on an object.
However, when the aperture mechanism is driven for purposes and applications such as those described above, in the case where the aperture is driven during live view, while capturing moving pictures, and so on, the state occurring while the aperture is being driven will appear in the display image, resulting in a luminance fluctuation spanning approximately one or two frames. As a result, the brightness of the image in the continuous display image will increase or decrease for an instant, resulting in a problem in that the continuous image will appear unnatural.
Various proposals have been made as measures to avoid such a problem, and Japanese Patent Laid-Open No. 11-112865 (called “Patent Document 1” hereinafter) discloses fixing the aperture during live view and carrying out exposure control using only the electronic shutter. Meanwhile, Japanese Patent Laid-Open No. 9-102905 (called “Patent Document 2” hereinafter) proposes driving the aperture outside of the charge accumulation period for the image sensor.
However, in the case where the aperture is not driven during live view, as in Patent Document 1, there is a problem in that the range in which the luminance of the object is tracked is limited.
In addition, there are multiple techniques for image sensor charge accumulation and accumulated data readout. For example, there is a technique in which the accumulation start and stop times are controlled all at once for the entire sensor, as with a CCD, and a technique in which charge accumulation and readout are carried out sequentially line by line and the actual exposure times differ at the top and bottom of the image, as with a CMOS sensor.
With an image sensor structured as a CMOS sensor, how the accumulation period is to be set is an issue when driving the aperture outside of the accumulation period as in Patent Document 2. With a structure such as that of a CMOS sensor, a charge is always being accumulated in a line somewhere in the sensor, depending on the electronic shutter speed; thus it is not easy to secure a non-exposure period.